Reproducing system and equalizing method for use in it

ABSTRACT

An object of the present invention is to increase the recording density of digital information signal in a recording medium without changing parameters of the recording medium and a RLL code. To accomplish the object, the present invention has a frequency response of the equalizer set so that an overall transmission frequency characteristic H(f) of the RLL coder, the recording medium, and the equalizer to an impulse becomes the one given by an equation below ##EQU1## where Fm denotes a cut-off frequency of the recording medium.

This application is a continuation of application Ser. No. 08/517,584,filed Aug. 22, 1995 now U.S. Pat. No. 5,602,816.

BACKGROUND OF THE INVENTION

The present invention relates to a recording and reproducing system forrecording and reproducing digital information signal in a recordingmedium such as CD and an equalizing method for use in it. Moreparticularly, it concerns a recording and reproducing system and anequalizing method for use in it that makes it possible to increaserecording density of digital information signal without changingparameters inherent to a recording medium.

Conventionally, the art of such a technique has been disclosed in, forexample, "Association of Television Technology," Vol. 42, No. 4 (April,1988), pp. 330-337. The prior art is described below.

A digital information signal to be recorded in a recording medium, suchas a CD, is modulated by a run length limited (hereinafter referred toas the RLL) coder with use of the socalled (d, k) RLL code of minimumrun length d and maximum run length k before being recorded. The signalreproduced from the recording medium is equalized by an equalizer beforebeing decoded to the original digital information signal by a decoder tofeed out.

FIG. 9 shows a frequency response M(f) of the RLL coder and a generaloverall transmission frequency response H(f) of the RLL decoder, therecording medium, and the equalizer. Curves (a) and (b) in the figureindicate the frequency response M(f) and the overall transmissionfrequency response H(f), respectively. Fb, Fn, and Fm in the figureindicate a recording bit rate after the modulation, a Nyquist frequency(=(Fb/d)/2), and a cut-off frequency of the recording medium 3,respectively. In general, the frequency response E(f) of the equalizeris set so that as shown by curve (b), the overall transmission frequencyresponse H(f) should become a raised cosine characteristic (100%roll/off factor) of a Nyquist frequency Fn. The recording bit rate Fbtherefore can be increased until bandwidth B (=2·Fn) of the overalltransmission frequency response H(f) become the cut-off frequency Fm.Hence, the recording bit rate Fb is given with respect to the cut-offfrequency Fm as

    Fb=d·Fm                                           (1)

FIG. 10 shows an eye pattern of the output signal of the equalizer 4with the minimum run length d being 3. If the recording medium 3 is anoptical disk, the cut-off frequency Fm of the recording medium 3 isgiven by an equation below: ##EQU2## where λ denotes a wavelength oflaser, NA is a numerical aperture of an optical system, and V is alinear velocity of the recording medium.

As an example, a CD system with the minimum run length d of 3 can bemade to have the recording bit rate Fb of around 4.32 Mbps and thebandwidth B of around 1.44 MHz. From Eq. 2, the cut-off frequency Fmbecomes around 1.44 MHz that is equal to the bandwidth B if thewavelength λ of the laser is 0.78 μm, the numerical aperture NA of theoptical system is 0.45, and the linear velocity of the optical disk is1.25 m/sec.

SUMMARY OF THE INVENTION

In general, the recording density of digital information signal can beincreased by way of changing parameters of the recording medium, such asthe wavelength λ of the laser of the optical disk, and parameters of theRLL code, such as minimum run length d. However, it has not been knownhow to increase the recording density further, or how to increase itwithout changing those parameters.

If it is tried to increase the recording density without changing theparameters of the recording medium and those of the RLL code with use ofthe prior art described above, the following problems arise, which aredescribed with reference to FIGS. 11 and 12.

In FIG. 11, curve (a') indicates a frequency response M(f) of the RLLcoder 2, curve (b') is an overall frequency response H(f), K is acoefficient of recording density increment, Fb'is a recording bit rate(=K·Fb), and Fn' is a Nyquist frequency (=(Fb'/d)/2=K·Fn). Of course,the cut-off frequency Fm of the recording medium is constant since theparameters of the recording medium and the RLL code are unchangeable.With respect to the prior art, the frequency response E(f) of theequalizer is set so that as shown by curve (b'), the transmissionfrequency response H(f) should become a roll-off raised cosinecharacteristic of a Nyquist frequency Fn'. That is, the bandwidth B ofthe transmission frequency response H(f) is limited by the cut-offfrequency Fm, and the roll-off factor β is decreased with thecoefficient K. Such relationships are given by

    B=(1+β)·Fn'=(1+β)·K·Fn

    Fm=2·Fn

Hence,

    ∴β=2/K-1                                      (3)

If the coefficient K in Eq. 3 above is 1.5, or if the recording densityis increased 1.5 times, the roll-off factor D is decreased to 33.3%. Ifthe minimum run length d is 3, the eye pattern of the output signal ofthe equalizer becomes the one as shown in FIG. 12, resulting in that anopen area ratio of the eye pattern is deteriorated to a great extent.

As described above, the prior art has the open area ratio of the eyepattern deteriorated if trying to increase the recording density withthe parameters of the recording medium and the RLL code made constant.This does not allow making a signal-to-noise ratio high. The system ofthe prior art therefore is likely affected by noises. That is, it isdifficult that the prior art increases the recording density furtherwith the parameters of the recording medium and the RLL code madeconstant.

In view of the foregoing, it is an object of the present invention tosolve the problems of the prior art to provide a recording andreproducing system that makes it possible to increase recording densityof digital information signal further without changing parameters of arecording medium and a RLL code and an equalizing method for use in it.

Briefly, the foregoing object is accomplished in accordance with aspectsof the present invention by the recording and reproducing system,comprising a RLL coder for modulating a digital information signal to berecorded in a recording medium with use of a run length limited codehaving a minimum run length d, an equalizer for equalizing a signalreproduced from the recording medium, and a decoder for decoding asignal equalized by the equalizer to the original digital informationsignal, wherein a frequency response of the equalizer is set so that atransmission frequency characteristic H(f) of the RLL coder, therecording medium, and the equalizer to an impulse becomes the one givenby Eq. 4 below ##EQU3## where Fm denotes a cut-off frequency of therecording medium that is given by Eq. 5 below ##EQU4##

With the frequency response of the equalizer is set as such, virtuallyall the isolated pulses are overlapped together from the minimum runlength d to the maximum run length k. This gives little interferenceamong the pulses even if the pulses come at random. The equalizingmethod of the present invention therefore provides sufficient open arearatio of eye pattern even with the recording density made high, or therecording density can be made high without lowering the signal-to-noiseratio.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be more fully described by reference to theaccompanying drawings in which:

FIG. 1 depicts a block diagram illustrating a construction for thedigital information signal recording and reproducing system of thepresent invention;

FIG. 2 depicts a graph illustrating frequency responses of theequalizing method of the present invention;

FIG. 3 depicts an example of eye pattern with use of the equalizingmethod of the present invention;

FIGS. 4(A&B) depicts an example of responses to isolated pulses with useof the equalizing method of the present invention as compared with thatof a prior art;

FIGS. 5(A&B) depicts a graph and a pattern illustrating another exampleof responded waveforms and eye pattern with use of the equalizing methodof the present invention;

FIGS. 6(A&B) depicts a graph and a pattern illustrating still anotherexample of responded waveforms and eye pattern with use of theequalizing method of the present invention;

FIGS. 7(A&B) depicts a graph and a pattern illustrating still anotherexample of responded waveforms and eye pattern with use of theequalizing method of the present invention;

FIGS. 8 (A&B) depicts a graph and a pattern illustrating still anotherexample of responded waveforms and eye pattern with use of theequalizing method of the present invention;

FIG. 9 depicts a frequency response of the equalizing method of theprior art;

FIG. 10 depicts an example of eye pattern with use of the priorequalizing method;

FIG. 11 depicts a frequency response of the prior equalizing method withthe recording density increased;

FIG. 12 depicts an example of eye pattern with use of the priorequalizing method with the recording density increased; and,

FIG. 13 depicts an block diagram illustrating an example of constructionfor the equalizer 4 shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following describes in detail a first embodiment according to thepresent invention by reference to the accompanying drawings.

FIG. 1 depicts a block diagram illustrating a construction for thedigital information signal recording and reproducing system that is anembodiment of the present invention. In the figure are shown an inputterminal 1, a run length limited (RLL) coder 2, a recording medium 3, anequalizer 4, a decoder 5, and an output terminal 6. In the figure, also,M(f), G(f), and E(f) indicate frequency responses of the RLL coder 2,the recording medium 3, the equalizer 4 to an impulse, respectively.Further, H(f) indicates an overall transmission frequency response ofthe RLL coder 2 to the equalizer 4 to the impulse.

In FIG. 1, the digital information signal entered from the inputterminal 1 is modulated by the RLL coder 2 with the (d, k) RLL code ofminimum run length d and maximum run length k before being recorded inthe recording medium 3. As an example, the RLL coder 2 of a digitalaudio system having a compact disk (CD) used as the recording medium 3employs a modulation method called the EFM (Eight-to-FourteenModulation) of which the minimum run length d is 3 and the maximum runlength k is 11.

A signal reproduced from the recording medium 3 is equalized by theequalizer 4. The equalized signal is decoded to the original digitalinformation signal by the decoder 5. The decoded digital informationsignal is fed out of the output terminal 6.

The present invention serves to set the frequency response E(f) of theequalizer 4 so that the overall transmission frequency response H(f) ofthe RLL coder 2 to the equalizer 4 to the impulse can be made as givenby Eq. 4 previously. FIG. 13 shows an example of construction of theequalizer 4.

In FIG. 13 are indicated delay lines 41a, 41b, 41c, and 41d that delaythe signal reproduced from the recording medium 3 by predetermined timesτ. The figure also shows multipliers 42a, 42b, 42c, 42d, and 42e thatmultiply by coefficients c the signal reproduced from the recordingmedium 3 and to the signals delayed through the delay lines 41a, 41b,41c, and 41d. The figure further shows an adder 43 that adds the signalsfrom the multipliers 42a, 42b, 42c, 42d, and 42e before feeding out tothe decoder 5. To set the frequency of the equalizer 4 constructed aboveso that the overall transmission frequency response H(f) can be made asgiven by Eq. 4, the coefficients c of the multipliers 42a, 42b, 42c,42d, and 42e and the delay times τ of the delay lines 41a, 41b, 41c, and41d should be adjusted appropriately depending on the frequency responseM(f) of the RLL coder 2 and the frequency response G(f) of the recordingmedium 3.

FIG. 2 depicts a graph illustrating frequency responses of theequalizing method of the present invention. A curve (c) represents theoverall transmission frequency response H(f). In the figure, d indicatesthe minimum run length of the RLL recording code, Fb' is a recording bitrate, Fn' is a Nyquist frequency, and K is a recording density incrementcoefficient. The overall transmission frequency response H(f), asdescribed previously, is the cosine characteristic represented by Eq. 4given before. It need hardly be said that the bandwidth B of the cosinecharacteristic is equal to the cut-off frequency Fm. The presentinvention sets the frequency response E(f) of the equalizer 4 so thatthe overall transmission frequency response H(f) can be made as shown bythe curve (c) in the figure.

FIG. 3 depicts an eye pattern illustrating the signal output of theequalizer 4 when the overall transmission frequency response H(f) isgiven by the curve (c) in FIG. 2. The eye pattern is an example when theminimum run length d is 3 and the recording density incrementcoefficient K is 1.5. Thus, we can see that the present invention canimprove the open area ratio of eye pattern to a great extent as comparedwith the usual ones.

FIG. 4 depicts graphs illustrating waveforms that have isolated waveresponses overlapped together from a 3T' pulse corresponding to theminimum run length d to a 11T' pulse corresponding to the maximum runlength k. FIG. 4(A) depicts a graph illustrating a waveform of theequalizing method of the present invention shown in FIG. 1. FIG. 4(B)depicts a graph illustrating a waveform of the previous equalizingmethod shown in FIG. 11. Comparing the waveforms of the both equalizingmethods up to t=0, that of the present invention shown in FIG. 4(A) hasvirtually all pulses from the 3T' pulse to the 11T' pulse overlappedtogether, but that of the previous equalizing method shown in FIG. 4(B)has not quite pulses overlapped. It is hardly necessary that thedifference is given in the eye patterns in FIGS. 3 and 12. Thus, theequalizing method of the present invention having all the isolatedpulses overlapped together gives little interference among the pulseseven if the pulses come at random. The equalizing method thereforeprovides sufficient open area ratio of eye pattern.

The example shown in FIG. 3 has the minimum run length d of 3 and therecording density increment coefficient K of 1.5, but the presentinvention is not limited to those values. The minimum run length d canbe made not less than 2, and the recording density increment coefficientK can be in a range of 1 to 2.

FIGS. 5 and 6 depict other graphs and patterns illustrating examples ofresponded waveforms and eye patterns when the recording densityincrement coefficient K is 1.25 and 1.75, with the minimum run length dof the both examples being 3. FIGS. 5(A) and 6(A) show the waveformsresponded to the isolated pulses. FIGS. 5(B) and 6(B) show the eyepatterns. For K=1.25 in FIG. 5, the open area ratio of eye pattern ishigh as the recording density increment coefficient K is small, causingno problems. For K=1.75 in FIG. 6, on the other hand, the open arearatio of eye pattern is low as the recording density incrementcoefficient K is large. The reason, as seen from the responses to theisolated pulses in FIG. 6(A), is that the pulses are overlappedtogether, but the whole pulse width are so extended that the level islowered. If the recording density increment coefficient K is wished tobe large, therefore, a signal-to-noise ratio of the recoding andreproducing system has to be made sufficiently high. The recordingdensity increment coefficient K of the present invention is limited bythe signal-to-noise ratio. However, as described above, if theminimum-run length d is 3, the present invention can achieve therecoding and reproducing system or recording medium that has therecording density improved up to around 1.75 times.

FIG. 7 depicts a graph and a pattern illustrating 10 another example ofresponded waveforms and eye pattern when the minimum run length d is 4and the recording density increment coefficient K is 1.5. FIG. 7(A)shows the waveforms responded to the isolated pulses. FIG. 7(B) showsthe eye pattern. As can be seen from the figure, all the isolated pulsesare overlapped together to provide a sufficient open area ratio of eyepattern. When the minimum run length d is 4, therefore, the recordingdensity can be increased up to around 1.5 times.

FIG. 8 depicts still another graph and pattern illustrating an exampleof responded waveforms and eye pattern when the minimum run length d is5 and the recording density increment coefficient K is 1.25. FIG. 8(A)shows the waveforms responded to the isolated pulses. FIG. 8(B) showsthe eye pattern. As can be seen from the figure, also all the isolatedpulses are overlapped together to provide a sufficient open area ratioof eye pattern. When the minimum run length d is 5, therefore, therecording density can be increased up to around 1.25 times.

As described so far, the present invention can increase the recordingdensity of the digital information irrespective of the recording mediumand modulating method.

Modifications other than described above can be made by those skilled inthe art without departing from the spirit and scope of the presentinvention. The above description of the preferred embodiment of thepresent invention and the preferred dimensions thereof are given by wayof example only and is not limited to the specific embodimentshereinbefore discussed but extends to all modifications. The scope ofthe present invention is defined in the claims appended hereto. Alldifferent modifications and changes made within the spirit and scope ofthe appended claims will be within the present invention.

What is claimed is:
 1. An information reproducing system comprising:an equalizer for equalizing a signal reproduced from a recording medium having a digital information signal recorded thereon and a decoder for decoding said equalized signal to an original digital signal; wherein said digital information signal recorded on said recording medium is a digital information signal modulated with use of limited run length code having a minimum run length of d and a maximum run length of k; and said equalizer equalizes a reproduced waveform from said recording medium so that rising or falling gradients of an isolated reproduced waveform of the minimum run length of d and the maximum run length of k are substantially same and a recording bit rate Fb of said digital information signal satisfies the equation below:

    Fb=K·d·Fm(K>1)

wherein Fm denotes a high cut-off frequency of said recording medium and K denotes a coefficient of the recording density.
 2. An information reproducing system according to claim 1, wherein said recording medium is an optical disk and sid minimum run length d is
 3. 3. An equalizing method comprising the steps of:equalizing a signal reproduced from a recording medium having a digital information signal recorded thereon with an equalizer; wherein said digital information signal recorded on said recording medium is a digital information signal modulated with use of a limited run length code having a minimum run length of d and maximum run length of k; and equalizing with said equalizer a reproduced waveform from said recording medium so that rising or falling gradients of an isolated reproduced waveform of the minimum run length of d and the maximum run length of k are substantially same and a recording bit rate Fb of said digital information signal satisfies the equation below:

    Fb=K·d·Fm(K>1)

wherein Fm denotes a high cut-off frequency of said recording medium and K denotes a coefficient of the recording density.
 4. An information reproducing system comprising:an equalizer for equalizing a signal reproduced from a recording medium having a digital information signal recorded thereon; and a decoder for decoding said equalized signal to said original digital signal; wherein said digital information signal recorded on said recording medium is a digital information signal modulated with use of limited run length code having a minimum run length of d and a maximum run length of k; and said equalizer equalizes a reproduced waveform from said recording medium so that the amplitude of each equalized waveform of said equalized signal of the minimum run length of d and the maximum run length of k substantially satisfies the formula below:

    cos (π/2·f/FM)

where f denotes a frequency of said each equalized waveform and Fm denotes a high cut-off frequency of said recording medium.
 5. An information reproducing system according to claim 4, Wherein said recording medium is an optical disk and said minimum run length d is
 3. 6. An equalizing method comprising the steps of:equalizing a signal reproduced from a recording medium having a digital information signal recorded thereon; wherein said digital information signal recorded on said recording medium is a digital information signal modulated with use of limited run length code having a minimum run length of d and a maximum run length of k; and said equalizing method equalizes a reproduced waveform from said recording medium so that an amplitude of each equalized waveform of said equalized signal of the minimum run length of d and the maximum run length of k substantially satisfies the formula below:

    cos(π/2·f/FM)

where f denotes a frequency of said each equalized waveform and Fm denotes a high cut-off frequency of said recording medium. 